This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-005741, filed Jan. 6, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a radio communication system such as a CDMA (code division multiple access) system, and more particularly, to transmission timing control of a control signal transmitted from a base station to mobile stations to establish synchronization between the base station and the mobile station.
In a CDMA system, in order to perform a simultaneous communication by a plurality of mobile stations, communication signals for the plural mobile stations are multiplexed by being spectrum-spread using a plurality of spreading signals. Each mobile station is assigned with its corresponding spreading signal. When a mobile station is communicating with a base station, the mobile station checks a quality of the control signal from adjacent base stations to prepare xe2x80x9chandoverxe2x80x9d. Each base station periodically transmits the control signal. The transmission timing of the control signal is selected among a plurality of slot timings. The control signal is also called a pilot signal.
When the mobile station receives the control signal from the base station, a first code (which is not spread) formed of a predetermined bit pattern and located at the head end of the control signal, as shown in FIG. 1, is first detected by a matched filter. The matched filter compares the received signal with the predetermined bit pattern. Since the control signal is periodically transmitted from the base station, each mobile station is informed of slot timings for transmission of the control signal by the base station, and detects the first code within the time of xc2x131 xcexcS from each of the slot timings.
After the first code (a start of the control signal) is detected and synchronization between the base station and the mobile station is established, the remaining portion (long code) of the control signal other than the first code, which was spread by a specific spreading signal by the base station, is despread by using the same spreading signal.
However, in such prior CDMA system, the control signals are transmitted from base stations at random (or asynchronously), as shown in FIG. 2. Accordingly, there are cases in which the control signals are transmitted from the plural base stations (e.g., BS1 and BS2) at the same time (slot timing xe2x80x9c3xe2x80x9d) with these signals overlapped. Therefore, when the mobile station receives the control signals from the plural base stations BS1 and BS2 simultaneously, the plural first codes are detected, as shown in FIG. 3. In a wireless communication, a signal has been transmitted via a reflection path as well as a direct path. Therefore, three first codes are detected by the matched filter.
It can not be specified from which base station the control signals related to the plural first codes detected are transmitted, based on the matched outputs. Accordingly, it is necessary to despread the remaining portion of the control signal other than the first code with the use of spreading signals of the plural base stations from which there is a possibility that the mobile station receives the control signals with trial-and-error.
This results in an increase in processing time for the reproduction of the control signal, thereby an increase in processing time for xe2x80x9chandoverxe2x80x9d using information included in the control signal from adjacent base stations has been a problem. Thus, in the conventional CDMA system, since the transmission timing of the control signal has been asynchronous or random between base stations, reproducing process have taken time.
Accordingly, it is an object of the present invention to provide a radio communication system or radio communication base station capable of reduction of the processing time for the reproduction of the control signal from the base station.
A related object of the present invention is to provide a control method for transmission timing of the control signal to be transmitted from the base station, by which the mobile station can reduce the processing time for the reproduction of the control signal.
According to the present invention, a transmission timing of the control signal is controlled such that each transmission timing differs between base stations which are adjacently located. Thus, the mobile station can reproduce the control signal using only a predetermined spreading signal for the base station which transmits the control single. Therefore the time for reproducing process of the control signal can be reduced.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.